1 battery or 2

I say, if you "really are committed" to running no alternator, you need to run higher voltage batteries. Cars don't run on 12 volts, they run at 14. But when you LOAD a battery, with fans and other fairly heavy draw, they don't even run at 12 or 14. So you probably should consider 16 volt battery(s)

This of course means you cannot use conventional (14V) regulated charger/ alternator to charge them